Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device is found in U.S. Pat. No. 5,475,318, which leverages thermal expansion to move a microdevice. A micro cantilever is constructed from materials having different thermal coefficients of expansion. When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,399. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
Micromachined MEMS devices have also utilized electrostatic forces to move microstructures. Some MEMS electrostatic devices use relatively rigid cantilever members, as found in U.S. Pat. No. 5,578,976. Similar cantilevered electrostatic devices are described in U.S. Pat. Nos. 5,258,591, 5,367,136, 5,638,946 5,658,698, and 5,666,258. The devices in the above patents fail to disclose flexible electrostatic actuators with a radius of curvature oriented away from the substrate surface. Other MEMS devices disclose curved electrostatic actuators. However, some of these devices incorporate complex geometries using relatively difficult microfabrication techniques. U.S. Pat. Nos. 5,629,565 and 5,673,785 use dual micromechanical substrates to create their respective electrostatic devices. The devices in U.S. Pat. Nos. 5,233,459 and 5,784,189 are formed by using numerous deposition and processing steps. Complex operations are required to create corrugations in the flexible electrodes. In addition, U.S. Pat. No. 5,552,925 also discloses a curved electrostatic electrode. However, the electrode is constructed from two portions, a thinner flexible portion followed by a flat cantilever portion.
Several of the electrostatic MEMS devices include an air gap between the substrate surface and the electrostatic actuator. The electrostatic actuators generally include flexible, curled electrodes. Typically, the gap starts at the beginning of the electrostatic actuator where it separates from the substrate surface and increases continuously along the length of the air gap. The size of the air gap increases as the actuator curls further away from the substrate surface along its length. The air gap separation between the substrate electrode and actuator electrode affects the operating voltage required to move the actuator. The larger the air gap or the higher the rate of increase, the higher the voltage required to operate the actuator. Further, due to manufacturing process and material variations, the size and shape of the air gap can vary substantially from device to device, making operation erratic. Traditional electrostatic devices have relatively large and variable operating voltage characteristics. It would be advantageous to be able to control the attributes of the air gap through the design of the electrostatic actuator. Lower operating voltage devices could be developed. The variation in operating voltage required for a given device could be minimized. Thus, more predictable electrostatic actuators could be developed. By properly designing the air gap in an electrostatic device, the operation of the device could be helped instead of hindered.
There is still a need to develop improved MEMS devices and techniques for leveraging electrostatic forces and causing motion within microengineered devices. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations in MEMS devices. These forces are readily controlled by applying a difference in voltage between MEMS electrodes, resulting in relatively large amounts of motion. Electrostatic devices operable with lower and less erratic operating voltages are needed. Advantageous new devices and applications could be created by leveraging the electrostatic forces in new MEMS structures.